1. Field of the Invention
The invention concerns a device for stabilising the inclination, relative to an external frame of reference, of a body mobile in rotation about at least one rotation axis relative to a support, the inclination of which relative to said external frame of reference fluctuates. It is more particularly, although not exclusively, concerned with stabilising an optical element mounted on board a vehicle, for example an aircraft (or a balloon, a space vehicle, etc) relative to which the optical instrument can rotate about two perpendicular rotation axes.
2. The Prior Art
The present invention concerns, for example, the input mirror of an observation telescope mounted on board an aircraft. As is well known, the function of this mirror is to reflect incident radiation along the input axis of the telescope, which axis is fixed relative to the aircraft (usually the axis of the telescope is parallel to the longitudinal axis and to the path of movement of the aircraft whereas the incident radiation is approximately transverse thereto). This mirror can be oriented about two orthogonal axes respectively perpendicular and parallel to the axis of the telescope. The problem is then to control the inclination of the mirror relative to the aircraft as accurately as possible so as to maintain the alignment of the reflected radiation with the axis of the telescope in spite of vibration of the aircraft and variations in its orientation in space.
Various stabilisation devices for one or two axes have already been proposed to meet this objective as closely as possible; some of them, for example, are described in documents FR-2.552.893, U.S. Pat. Nos. 3.378.326, 3.493.283, 4.155.621 and 4.576.449. More generally speaking, there is also known from the document U.S. Pat. No. 3.499.332 or the document SU-783.587 a device for stabilising an inertial platform. All these systems employ gyroscopes or inertia wheels.
There is a particular problem to be overcome in achieving control relative to two axes because the variations in the inclination of the aircraft relative to a transverse axis perpendicular to the input axis of the telescope requires a correction to the mirror angle in a ratio of 1/2 whereas in the event of variation in the inclination relative to the longitudinal axis the angle correction has to be performed with a unity ratio.
Also, the observation of mobile targets often makes it essential to be able to aim an input mirror of this kind at high speed.
Problems of the same kind are associated with the use of other optical instruments, for example the orientable reflector of a laser mounted on board a vehicle, usually an aircraft or space vehicle.
The stabilisation devices known at this time do not enable highly accurate stabilisation (to within one minute of angular arc or even less) in combination with aiming at high speed (for example at rates up to 200.degree./s) if an inertial reference system is employed.
Thus devices using a stabilised platform are not suited in practice to angle correction with the aforementioned 1/2 ratio.
Devices incorporating gyroscopes linked to the instrument to be stabilised do not allow high aiming speeds since this requires a high degree of precession to be imposed on the gyroscope, which compromises their accuracy. Since for aiming about a transverse axis the precession has to be in a ratio of 1/2 to the aiming angle, additional error results.
It must be borne in mind that precession is a motion resulting from an external torque which is perpendicular to the plane defined by the spin axis and the axis of the disturbing torque.
Generally speaking, the known solutions are concerned with small aiming devices with no automatic control system.